1. Field of the Invention
The present invention relates to vertical pulverizers, and more particularly, to guide bushings for tensioning rods of vertical pulverizers.
2. Description of Related Art
A variety of devices and methods are known in the art for pulverizing raw materials. Of such devices, many are directed to pulverizing coal for use as a fuel. In coal-fired furnaces, for purposes of improved and more efficient ignition, it is preferred to pulverize the coal to a fine powder before introducing it into the furnace for combustion. Coal pulverization involves systematically comminuting coal to a desired, preferably optimum size, e.g. a fine powder, prior to introduction into a coal-fired furnace. Currently, coal pulverization systems include ball tube type mills, high-speed attrition type pulverizers, and vertical roller type mills.
Ball tube type mills are low speed mills that have their origins in the 1930's and 1940's. The ball tube type mill comprises a plurality of hardened steel balls that are disposed in a large, rotatable barrel. While the barrel rotates, coal is introduced into the barrel ends. Through the rotating action of the barrel, the steel balls fall and cascade onto the coal, pulverizing the coal by the impact of the cascading steel balls. The pulverized coal is then removed and fed into a coal-fired furnace. Ball tube type mills are successfully used in conjunction with highly abrasive coal. Ball tube mills rotate at approximately 20 RPM.
A high speed attrition type pulverizer typically, as in the ATRITA® pulverizer available from Riley Power Inc. of Worcester, Mass. (U.S. Pat. Nos. 7,172,146 and 7,028,931) provides three stages of pulverization. Each stage is powered by a common rotary assembly. Coal enters the first (crushing) section where a plurality of rotating and reciprocating swing hammers crush the coal against a grid. The grid deters passage of coal that has not been crushed sufficiently to a preferred nominal size, e.g. about ¼ inch. Once the coal has been reduced to a nominal size, it passes through a grid section and then is introduced into the second section where coal particles are forced to rub together by a set of impellers on a rotating disk, further reducing the coal size. Next, it enters a section where it is forced between a set of high speed rotating pegs and stationary clips. Then the coal exits through a rejecter assembly (while coarse particles are forced back into the previous section for further size reduction), to the final third section. The third section is an exhauster section which transports the fine, pulverized coal in a fluid stream to the coal-fired furnace.
Vertical roller type mills pulverize coal on a rotating grinding table. A plurality of rollers typically cast in abrasion resistant material apply a shearing force downward onto the carrier table and thus apply a grinding pressure to the coal. On the top of the carrier table is mounted a set of segments cast from a similar abrasion resistant material. The pulverized coal is then removed from the mill using a high velocity stream of air and fed into a coal-fired furnace. Vertical pulverizers' rotating tables typically turn at approximately 25 RPM.
Loading the grinding rollers in most vertical coal pulverizers has typically been via hydraulic cylinders. Hydraulic cylinders are used as a tool to preload tensioning rods which act on the roller assemblies to transfer the grinding pressure onto the coal and grinding table. The tensioning rod must be free to move either way in the vertical direction to load and unload the system. A guide bushing around the tensioning rod provides a bushing surface to accommodate this vertical movement. Typically, the guide bushing is located inside a doghouse structure which provides protection for the guide bushing and other loading assembly components held within from factory contaminants. The doghouse also encloses part of the seal air system. Previously known guide bushings are unitary structures that must be mounted over an end of the tensioning rod. Typical guide bushings consist of several cast or forged pieces welded together. When a typical guide bushing or tensioning rod needs to be replaced due to wear or failure, the procedure for replacement is difficult and time consuming. The end of the tensioning rod must be freed so the guide bushing can be removed over the end of the tensioning rod. This involves removing the doghouse structure and transferring the load of the guide frame, classifier and/or other massive components so that the tensioning rod can be freed. Approximately three to five days of downtime can result when replacement of a guide bushing is required.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there still remains a continued need in the art for a guide bushing that is easier to remove and/or install about a tensioning rod. There also remains a need in the art for such a guide bushing that is easy to make and use. The present invention provides a solution for these problems.